Broadcasters Flag Problems With FCC Wireless Interference Plan

WASHINGTON—Last week, the last group of comments
about methodology proposed by the FCC’s Office of Engineering and
Technology for determining interference between TV broadcasters and broadband
wireless was made public on the FCC’s Electronic Comment Filing
System. While comments from the wireless side generally supported the FCC’s
approach, the National Association of Broadcasters, in the largest filing about
the proceeding, disassembled it piece-by-piece, showing how it was
fundamentally flawed and how its use would be in violation of the law Congress
passed allowing the Incentive Auctions.

With few exceptions, the wireless industry accepted the FCC’s analysis,
although most players stated that it was imperative the FCC clearly reveal any
interference impairments in the forward auction. Their support for the FCC
proposal isn’t surprising, considering it was skewed to provide the most
spectrum for auction--even if this provided less protection to TV viewers--by
using unrealistic assumptions about LTE base station facilities and applying
clutter loss to interfering wireless signals.

Some commenters from the wireless industry argued that the FCC should use the
F(50.10) probability for analyzing interference from DTV to wireless LTE base
stations instead of the proposed F(50,50) probability, explaining that even if LTE
base stations had more options when it came to reducing interference, having
that interference present 50 percent of the time would impair service from the
base station.

Sprint’s filing, at 30 pages, was the largest next to NAB’s 50-page filing.
That company provided maps showing that “the Longley-Rice model yields
remarkably similar results from those derived from the Hata model for the
markets studied.” Sprint’s maps show coverage from WPBT in Miami, Fla.

Longley-Rice is a terrain-sensitive model, while Hata does not
include terrain in calculations, but does include urban clutter. It isn’t
surprising that the two models match in South Florida, given there is really no
terrain to speak of. The results would be a different if Hata had been used to
generate the coverage map of WCBS-TV that Sprint used later in its comments to
justify use of the F(50,10) instead of F(50,50) probabilities for DTV
interference to base stations. The impact of terrain is obvious in the WCBS-TV
map.

In its comments, Sprint said: “the commission should not bar auction of
spectrum in areas where desired to undesired (D/U) ratios are minimally
exceeded.” Sprint suggested wireless operators could use directional antennas
or reduced power to operate in these areas. It also suggested raising the
allowed interference to TV viewers from 0.1 to 0.5 percent, the same level used
for interference between TV stations. These comments might make sense if the
service areas and interference were modeled based on LTE stations exactly as
they would be deployed instead of in a 10-km grid with reduced antenna height
and reduced power. The errors inherent in the assumptions used in predicting
the interference from LTE base stations are so large as to completely mask any
distinction based on tenths of percent of interference.

CTIA, the Wireless Association, raised concerns about use of Longley-Rice for
modeling propagation from LTE base stations in its comments, stating: “CTIA
also believes that use of the Longley-Rice radio propagation model is
appropriate for modeling the effects from DTV transmissions. However, CTIA
believes that other propagation models may be better suited for modeling the
propagation losses from wireless LTE systems given the inherent operational
differences between LTE and high powered broadcast television transmissions.
CTIA encouraged the Commission to continue exploring which technical
assumptions will best and most accurately model the potential interference
environment in the 600 MHz band.”

CTIA emphasized the information generated from this approach must be publicly
disclosed “with sufficient granularity to permit a thorough assessment of every
grid point.”
CTIA further stated: “Such data will be necessary for wireless bidders to be
able to assess whether, how, and to what extent it may be possible to mitigate
the interference and degree of impairment to the license. Only then can a
bidder assign a value to a license area containing the grid points at issue.”
CTIA noted that the data must include the number and identity of any
interferers, whether the interference is to uplink or downlink LTE blocks, the
interference field strength limits for DTV into wireless, to what extend a DTV
contour overlaps the license area and to what extend restricted areas within
the license area would be subject to exclusion zones due to the presumed
presence of one or more base stations that would cause harmful interference to
a TV station.

Would it be unreasonable for broadcasters to ask for a similar analysis for
each of the blocks in their coverage area?

If the results are in the same form as currently used in TV study, that data
would be available. There is, however, one problem: that data is useless.

The wireless operator can determine exactly what interference will be caused by
a TV station at any point as the TV station’s location, antenna pattern antenna
height and operating power level are all known. A TV station can’t accurately
determine what viewers would be impacted by LTE interference because the
analysis, as proposed, is based on hypothetical LTE base stations that, as the
FCC admits, are positioned on a large grid to ease computation time, and as NAB’s
comments show, don’t come close to matching real world deployments.

The NAB, ABC Television Affiliates Association, FBC Television
Affiliates Association, CBS Television Network Affiliates, NBC Television
Affiliates, the Association of Public Television Stations, the Corp. for
Public Broadcasting, and the Public Broadcasting Service all filed joint
comments stating: “The proposed OET Methodology relies on clearly erroneous
assumptions and inputs that significant underestimate inter-service
interference.”

Broadcasters point out that the OET methodology assumes that wireless base
station antennas (receive and transmit) have a height of 100 feet and operate
with an ERP of 720 Watts or 120 Watts per MHz. However, the FCC’s
proposed rules for the 600 MHz allow heights up to 305 meters with 1,000 Watts
per MHz--significantly higher than antenna heights used for the interference
analysis.

The NAB examined average tower heights of existing wireless facilities using
American Tower’s National Site List. Of the 704 ATC wireless facilities in
Alabama, only 19 are listed at 100 feet or less--the height the FCC would use
for interference analysis. More than 500 sites have tower height exceeding 200
feet above ground, and the average tower height in Alabama is 247.4 feet. The
story is the same in other markets--only 15 of 336 wireless facilities in
Maryland are 100 feet or less and almost a quarter of them exceed 200 feet. The
average tower height in that state is 192.5 feet. The NAB also cited tower
heights in New Jersey and Pennsylvania to show the vast majority of wireless
sites exceed 100 feet.

The NAB stated: “Thus, the OET methodology makes assumptions inconsistent with
both commission’s proposed rules and readily ascertained information
about actual wireless service deployment. In both cases, OET’s
erroneous assumptions are slanted towards predicting less interference than
would likely actually be observed.”

The organization suggested that the commission either align the service rules
for 600 MHz operation with the OET methodology assumptions, or adjust the
parameters used in the methodology to be consistent with the proposed rules.

This isn’t the only place where the assumptions used in the OET methodology don’t
match FCC rules. Broadcasters that want to use multiple transmitters in a
distributed transmission system have to calculate interference to other
broadcasters based on the root-sum-square method to combine signals from
multiple transmitters when calculating adjacent channel and co-channel
interference. Signals from two transmitters, even if the signal from each one
of the individual transmitters falls below the threshold for causing
interference, must be combined to see if together they will cause interference.

This simple concept has been ignored in the OET methodology. The impact of
multiple interfering signals is not considered. The NAB provided an example to
highlight how each of two signals could meet the 15 dB D/U threshold for
interference, but when combined would result in interference at the receiver.

The NAB stated: “The commission has a statutory responsibility to protect
broadcasting and DTV viewers and cannot ignore ‘real world’ interference from
multiple wireless operations; thus, any methodology used to predict potential
inter-service interference should take this into account. The Joint
Broadcasters urge the commission to consider interference from multiple wireless
base stations using either a simple direct summation method or the RSS method
utilized for calculating interference from multiple DTS transmitters under the
current rules.”

The NAB pointed out that the Public Notice requesting comment acknowledges
wireless facilities in practice can be located every few fractions of a
kilometer in dense urban markets. The OET methodology not only uses height and
powers below those allowed by proposed 600 MHz service rules and ignores
contribution of multiple transmitters, but it bases the interference analysis
on a 10 km grid of these under-powered, low- height base stations. The problem
with this approach should be obvious.

The NAB noted that according to the Public Notice, this 10 kilometer spacing
was based primarily on computation limitation and not on modeling practical
interference issues associated with real world deployments.

The NAB commented that: “While the Joint Broadcasters are sensitive to
computational limitations associated with using the complex OET methodology,
the appropriate solution to that problem is to use a simpler methodology-- not
to make demonstrably false assumptions that will underestimate the potential
for inter-service interference.”

Both the NAB and broadcasters argue that the OET methodology would “inaccurately
and artificially” reduce the predicted impact of interference from wireless
operators to television broadcast by incorrectly apply clutter factors. The
argument continued that such methodology would add a 5 to 8 dB clutter loss to
the interfering wireless signal, which would add to the errors already
introduced by assuming low power and low height and ignoring multiple
transmitters.

The group of broadcasters further noted: “To suggest, for example, that all of
the trees in an area classified as ‘Forest Land,’ or all of the buildings in an
area characterized as ‘Residential’ would be aligned so as to block the
undesired interfering signals from multiple wireless base stations to
viewers’ DTV antennas, but those same DTV antennas would be
completely free of any obstruction to the desired DTV signals, is simply
irrational. Clutter should not be considered on the interfering wireless signal
to DTV viewers’ home reception.”

The NAB noted that the OET staff indicated the use of clutter was based on OET
Bulletin 73 and the Public Notice cites that as a reference. The problem is
that OET-73 was designed for point-to-point analysis to determine if a specific
location is served. For the proposed interference methodology, the same clutter
loss would be applied to interference over the entire 4 km square area in the 2
km grids OET proposes uses for determining interference to DTV reception. NAB
provided a map showing clutter can vary significantly even over short
distances.

The analysis OET used to determine appropriate D/U ratios when calculating interference between LTE OFDM transmissions and DTV
reception is likely to be wrong. It hasn’t been tested. While testing has been
done on interference between NTSC and ATSC signals and between two ATSC
signals, the NAB noted there is no substantial body of testing on the
co-channel characteristics of LTE OFDM based signals into ATSC receivers.

Depending on receiver design, some parts of the 8-VSB spectrum are more
susceptible to interference, meaning the amount of interference won’t be
proportional to the amount of overlap. For example, receivers relying on the pilot
carrier will be more susceptible to interference in the lower one MHz of the
channel.

In its comments, the NAB presented maps showing the areas gained by using the
FCC’s overly complex and inaccurate methodology are minuscule. In one map that
showed the difference between the OET and contour based approaches, the NAB
noted: “To the extent that certain ‘white spaces’ are present within the
contour, these generally represent mountain ranges with little or no population
to be served. They certainly do not represent areas where widespread wireless
deployment is possible or would be anticipated.”

The NAB said: “These two examples suggest that a straightforward contour
calculation does not produce meaningful inefficiencies as compared to the
unduly complex and computationally intensive OET methodology. Contrary to
OET’s assertions, a separation distance approach is not spectrally
inefficient in any meaningful sense, and the proposed methodology will not
produce significant additional spectrum.”

There is much more detail and other examples in the Comments of the NAB, ABC Television Affiliates Association, et al.

I don’t see how the OET can justify adoption of such a flawed methodology--one
that gives precise but bogus results. I hope the policy makers and economists
at the FCC don’t try to put the engineers at the OET, who understand the
complexity of the analysis and know the limitations of the input data, in the
uncomfortable position of justifying such a flawed methodology when there are
alternatives--as the NAB indicates--that are much simpler. If the areas defined
for auction are to be trusted by both broadcasters and bidders, they can’t be
built on input data that doesn’t reflect the real world.

While the wireless industry supported the OET methodology, it wasn’t without
caveats, and this is even after OET weighted the analysis in their favor, as
NAB explained. Rather than waste more time and money trying to refine the
proposed methodology to the point it can be trusted, if that is possible, it
would be better to put the effort into an alternative proposal broadcasters and
bidders accept.